1. Field of the Invention
This invention relates to a radiation image read-out apparatus wherein a stimulable phosphor sheet, on which a radiation image has been stored, is exposed to stimulating rays, which cause the stimulable phosphor sheet to emit light in proportion to the amount of energy stored thereon during its exposure to radiation, and the emitted light is photoelectrically detected and converted into an image signal representing the radiation image.
2. Description of the Prior Art
When certain kinds of phosphors are exposed to radiation such as X-rays, .alpha.-rays, .beta.-rays, .gamma.-rays, cathode rays or ultraviolet rays, they store part of the energy of the radiation. Then, when the phosphor which has been exposed to the radiation is exposed to stimulating rays such as visible light, light is emitted by the phosphor in proportion to the amount of energy stored thereon during its exposure to the radiation. A phosphor exhibiting such properties is referred to as a stimulable phosphor.
As disclosed in U.S. Pat. Nos. 4,258,264, 4,276,473, 4,315,318, 4,387,428, and Japanese Unexamined Patent Publication No. 56(1981)-11395, it has been proposed to use stimulable phosphors in radiation image recording and reproducing systems. Specifically, a sheet provided with a layer of the stimulable phosphor (hereinafter referred to as a stimulable phosphor sheet) is first exposed to radiation which has passed through an object, such as the human body. A radiation image of the object is thereby stored on the stimulable phosphor sheet. The stimulable phosphor sheet is then scanned with stimulating rays, such as a laser beam, which cause it to emit light in proportion to the amount of energy stored thereon during its exposure to the radiation. The light emitted by the stimulable phosphor sheet, upon stimulation thereof, is photoelectrically detected and converted into an electric image signal. The image signal is then used during the reproduction of the radiation image of the object as a visible image on a recording material such as photographic film, on a display device such as a cathode ray tube (CRT) display device, or the like.
Radiation image recording and reproducing systems which use stimulable phosphor sheets are advantageous over conventional radiography using silver halide photographic materials, in that images can be recorded even when the energy intensity of the radiation to which the stimulable phosphor sheet is exposed varies over a wide range. More specifically, since the amount of light which the stimulable phosphor sheet emits when being stimulated varies over a wide range and is proportional to the amount of energy stored thereon during its exposure to the radiation, it is possible to obtain an image having a desirable density regardless of the energy intensity of the radiation to which the stimulable phosphor sheet was exposed. In order to obtain the desired image density, an appropriate read-out gain is set when the emitted light is being detected and converted into an electric signal to be used in the reproduction of a visible image on a recording material, such as photographic film, or on a display device, such as a CRT display device.
In the radiation image recording and reproducing systems described above, a radiation image read-out apparatus is used in order to detect an image signal from a stimulable phosphor sheet, on which a radiation image has been stored. In the radiation image read-out apparatus, a stimulable phosphor sheet, on which a radiation image has been stored, is exposed to stimulating rays, which cause the stimulable phosphor sheet to emit light in proportion to the amount of energy stored thereon during its exposure to radiation, and the emitted light is photoelectrically detected and converted into an image signal representing the radiation image. It is necessary that the image signal obtained from the radiation image read-out apparatus accurately represents the radiation image, which was stored on the stimulable phosphor sheet.
By way of example, when radiation is uniformly irradiated to a stimulable phosphor sheet, the stimulable phosphor sheet is then exposed to stimulating rays, which cause it to emit light in proportion to the amount of energy stored thereon during its exposure to the radiation, and the emitted light is photoelectrically detected and converted into an image signal, the image signal should have uniform value. However, it often occurs that the value of the image signal thus detected is not uniform. The reasons why such a problem occurs will be described hereinbelow.
With a certain radiation image read-out apparatus, a stimulable phosphor sheet, on which a radiation image has been stored, is scanned with a light beam (stimulating rays) in a main scanning direction and a sub-scanning direction which is approximately normal to the main scanning direction. When each portion of the stimulable phosphor sheet is thus exposed to the light beam, the exposed portion emits light in proportion to the amount of energy stored thereon during its exposure to the radiation. The emitted light is detected and converted into an image signal by a photoelectric conversion means, which has a light receiving surface, positioned to extend along the main scanning line on the stimulable phosphor sheet. (By way of example, the photoelectric conversion means is constituted of a combination of a light guide member with a photomultiplier. The light guide member has a light receiving surface, positioned to extend along the main scanning line on the stimulable phosphor sheet, and a ring-shaped light output face. The photomultiplier receives the light emanating from the light output face of the light guide member. Alternatively, as disclosed in, for example, U.S. Pat. No. 4,864,134, the photoelectric conversion means may be constituted of a long photomultiplier, which has a long light receiving surface, positioned to extend along the main scanning line, and which is located close to the stimulable phosphor sheet.) In such a radiation image read-out apparatus, even if the amount of light, which is emitted by the stimulable phosphor sheet and which impinges upon the light receiving surface of the photoelectric conversion means, is the same, the sensitivity with which the photoelectric conversion means detects the light varies in accordance with the position, with respect to the main scanning direction, from which the light impinges upon the light receiving surface. The nonuniformity in the sensitivity, with which the photoelectric conversion means detects the light emitted by the stimulable phosphor sheet, is referred to as shading.
Also, by way of example, in cases where a rotating polygon mirror, which has a plurality of reflecting surfaces, is used to scan the light beam in the main scanning direction, problems with regard to surface inclination occur due to, primarily, the inclination of the rotation axis of the rotating polygon mirror. Specifically, even if the light beam impinges upon the rotating polygon mirror from the same direction, the directions along which the light beam is reflected by different reflecting surfaces of the rotating polygon mirror deviate in the sub-scanning direction. In such cases, the intervals, at which a plurality of scanning lines of the light beam are formed on the stimulable phosphor sheet, do not become uniform with respect to the sub-scanning direction. As a result, part of a scanning line (e.g. a 10.mu.m-wide part of a scanning line formed with a light beam having a beam diameter of 100.mu.m) overlaps the adjacent scanning line, or two adjacent scanning lines become spaced too far apart from each other. Therefore, when a visible image is reproduced from the image signal detected from the stimulable phosphor sheet, nonuniformity in the image density occurs in the sub-scanning direction with a period corresponding to one turn of the rotating polygon mirror.
In order for the shading, the nonuniformity in the image density, or the like, to be eliminated, a novel method has been proposed in, for example, U.S. patent application Ser. No. 391,830, now U.S. Pat. No. 4,985,629. With the proposed method, a stimulable phosphor sheet is uniformly exposed to radiation in a radiation image recording apparatus. The stimulable phosphor sheet, which has been uniformly exposed to the radiation, is then exposed to a light beam, which causes the stimulable phosphor sheet to emit light in proportion to the amount of energy stored thereon during its exposure to the radiation, and the emitted light is photoelectrically detected. In this manner, the characteristics of the shading along the main scanning direction are detected with respect to the respective reflecting surfaces of the rotating polygon mirror. (The characteristics of the shading include not only the effects from the nonuniformity in the sensitivity, with which the photoelectric conversion means detects the light emitted by the stimulable phosphor sheet, but also the effects from the nonuniformity in the image density, differences in reflectivity among the reflecting surfaces of the rotating polygon mirror, or the like. All of these sources, from which the problems occur in that an image signal having uniform value cannot be obtained from a stimulable phosphor sheet when the stimulable phosphor sheet is exposed uniformly to radiation, will hereinbelow be referred to as the characteristics of the shading.) The information about the characteristics of the shading is stored in a storage means. When an image signal representing a radiation image is detected from a stimulable phosphor sheet, on which the radiation image has been stored, the image signal is corrected in accordance with the shading characteristics stored in the storage means.
However, it often occurs that, for example, the extent of the surface inclination of the rotating polygon mirror changes during the operation of the radiation image read-out apparatus. As a result, the shading characteristics change. Therefore, in order that image signals accurately representing radiation images may be obtained, it is necessary for operations to be carried out periodically wherein a stimulable phosphor sheet is uniformly exposed to radiation in a radiation image recording apparatus and an image signal is detected from the stimulable phosphor sheet, which has been uniformly exposed to the radiation, with the radiation image read-out apparatus. However, considerable time and labor are required to carry out such operations. Also, it is not easy to irradiate radiation uniformly to the stimulable phosphor sheet. It is possible for the manufacturer of the radiation image read-out apparatus to detect the shading characteristics by uniformly exposing the stimulable phosphor sheet to radiation in a radiation image recording apparatus in the course of assembling the radiation image read-out apparatus. However, from the point of view of the required time and labor and the accuracy with which the shading characteristics are detected, it is difficult to have the user of the radiation image read-out apparatus periodically detect the shading characteristics.